Tuners for 1.3 GHz Elliptical Cavities Warren Schappert Fermilab TD/I&C Thursday May 29, 2013

Cavity Detuning SC cavities – Operate with very narrow bandwidths – Manufactured from thin sheets of niobium to allow cooling – Resonance frequency sensitive to mechanical distortions of cavity walls Tuner needed to – Tune cavity resonance to RF frequency following cool-down – Compensate for dynamic detuning Lorentz force Helium pressure variations Microphonics – Tune cavities off resonance in case of failure Image: Maurisz Grecki

Mechanical Cavity Tuning Variety of tuning methods – Mechanical – Reactive – Pressure Previous surveys by Ed Daley and Stefan Simrock – – Mechanical tuner most common tuner for 1.3 GHz cavities consisting of – Stepper Motor – Reduction Gearbox – Rotational-to-Linear Conversion Mechanism – Piezo Stack Actuator

Tuner Performance Requirements Range – Static Tuner Sufficient to compensate for variations in resonance frequency following cool-down, e.g. 0.5 MHz Shift cavity off resonance by many (>20) bandwidths in the event of failure – Dynamic Tuner Greater than maximum cavity dynamic detuning from all sources (e.g. 1kHz) Precision – Static Small fraction of fast actuator range (e.g. 100Hz/1kHz) should be adequate for normal operation Small fraction of a bandwidth (e.g. 10Hz/200 Hz) to provide for tuning in event of fast actuator failure – Dynamic Small fraction of a bandwidth (e.g. <10Hz/200 Hz) Stiffness High stiffness – Bulk of actuator force should be transmitted to the cavity Lash and Hysteresis – No lash during dynamic operation Lash and hysteresis during static tuning may be acceptable Reliability and Lifetime – 10 yr operation  30M motor steps, 10 9 piezo cycles Specifications must be tailored to specific cavity designs – e.g. Spring constant, sensitivity, df/dP, etc. Example FNAL Functional Requirements

S1G Tuner Comparison S1 Global Cryomodule at KEK built with 4 distinctly different cavity/tuner types KEK Cavity/KEK Slide Jack-Central Mount KEK Cavity/KEK Slide Jack-End Mount DESY Cavity/Saclay-DESY Tuner FNAL Cavity/INFN Blade tuner – /papers/mooda02.pdf Different design tradeoffs for each type Saclay-DESY tuner based on proven design in use at TTF KEK cavity/tuner very stiff to minimize dynamic detuning on flattop INFN blade tuner light and low cost – Unique opportunity for back-to-back performance comparison

Saclay Lever Tuner Compound lever mechanism acting at one end of the cavity Saclay design – Further development at DESY In current use at TTF

KEK Slide Jack Tuner Ramp mechanism End or central mount External stepper motor

INFN Blade Tuner Flexure mechanism Central coaxial mount

Tuner Operation Static – Cool-down Relax tuner to unload cavity during cool-down – Operation Run motor until cavity resonance within specified tolerance band wrt RF frequency Dynamic – “Standard” approach Drive piezo with half sine pulse prior to RF pulse Tune pulse parameters to minimize detuning during flattop – First demonstrated at DESY – Adaptive algorithms FNAL, DESY,… – LFD compensation during long RF pulses – Both work well for RF pulses short with respect to period of dominant mechanical modes “Standard” performance degrades for longer pulses – Adaptive algorithms able to automatically compensate for He pressure variations and other sources of long term drift

Dynamic Tuner Performance Routinely reduce LFD from several hundreds of Hz to 10 Hz or better in FNAL/NML/CM1 S1G Performance comparable for all four different designs tested FNAL/NML/CM1

Feed-forward Resonance Stabilization in Pulsed Cavities Cavities sensitive to changes in He pressure df/dP  50 Hz/Torr – Can lead to large shifts in resonance frequency Adaptive algorithm can adjust piezo bias based on running average of detuning during previous pulses – Resonance can be stabilized to better than 1Hz on average Residual pulse-to-pulse detuning (microphonics) small in FNAL/NML/CM1 – Lower in the middle ( – Higher at the ends (9 Hz) Vacuum pumps Microphonics compensation requires feedback KEK/S1 Global FNAL/NML/CM1

Microphonics in CW Cavities Microphonics extensively studied in CW cavities HoBiCaT 1.3 GHz CW – CW detuning dominated by He pressure variations Can be controlled to 1 Hz RMS or better Same level may not be attainable in pulsed cavities – Accurate detuning measurements possible only when RF pulse is present HoBiCaT  Gaussian =0.82 Hz FNAL

Narrow Bandwidths and Long Pulses For some applications longer pulses and narrower bandwidths may be useful – 2011 Test using 9ms pulses in FNAL/NML/CM1/C5 and C6 for Project X Q L ranged between 3x10 6 and 3x10 7 – Adaptive LFD control able to limit detuning to better than 50 Hz across the flattop and most of the fill FNAL/NML/CM1/C5 and C6

Tuner Reliability Slow and fast tuners must operate reliably over the lifetime of the machine – Failure prone components should be located outside vacuum vessel if possible Heat leaks Linkage spring constant and inertia – Internal components must have high MTBF Stepper motor, Gearbox, Linkage, Piezo… – Access ports for LRU repair or replacement Experience varies widely – Notable large scale “Piezo” failures at SNS – Only a handful of tuner failures at DESY/FLASH – Experience at FNAL shows there can be a long learning curve even with a “proven” design Need to treat tuner design like “Rocket Science” – Many of the same components used by space flight community Similar environmental and reliability requirements

Piezo Actuator Reliability Piezo actuators can be extremely reliable if treated properly – High piezo cryogenic lifetime demonstrated in several tests INFN – 1.5x10 9 cycles  10 yr operation NASA – cycles Can fail quickly if not treated properly – Piezo lifetime strongly affected by Humidity, temperature and voltage Cryogenic vacuum should be close to ideal environment – Shear forces can lead to rapid failure Encapsulation critical – Careful encapsulation design “Piezo” failures at SNS caused by 1 bar pressure transients – Capsule failed not actuator SNS FNAL

Stepper Motor Reliability Stepper motor warm lifetime dominated by bearing wear – Operation in cold vacuum requires bearing and other modifications Vacuum and space qualified motors commercially available Phytron:65Msteps DESY: 65MSteps

Limits to Tuner Reliability Cold vacuum is difficult environment for electromechanical systems – Every component is a potential point of failure, piezo, stepper, gearbox, linkage… Reliable tuner design requires careful component selection and extensive warm and cold testing of individual components and of assembled tuners – e.g. DESY experience with motors – Suitable high-reliability components are commercially available Tuner reliability program should be initiated and completed well prior to commencement of procurement and production – Once production begins schedule will take priority over everything else Life Test Failure of Harmonic Gears in a Two-Axis Gimbal for the Mars Reconnaissance Orbiter Spacecraft Michael R. Johnson*, Russ Gehling**, Ray Head+

Tuner Cost AES ILC Cost Study – commissioned by Fermilab 2007/2011 – Tuning 4% of overall cryomodule cost

Summary Variety of tuners for 1.3 GHz elliptical cavities have been built and tested – Four distinctly different candidate cavity/tuner designs compared in ILC/S1G tests at KEK in 2010 Each demonstrated excellent performance following compensation Variety of control algorithms have been implemented and tested – Feed-forward LFD compensation and resonance stabilization against He pressure variations are now well understood – At narrower bandwidths microphonics becomes more important Feedback may be useful – No fundamental reason detuning from all sources can’t be controlled to 1Hz or better Tuner represents only a few percent of overall cryomodule cost Selection of tuner for collider cavities should focus on lifetime and reliability – Required reliability can be achieved Needs careful engineering of each element in the electro-mechanical chain Needs extensive cold testing of each component and of entire tuner assembly Tuner reliability evaluation and improvement program should be initiated and completed well before commencement of procurement and production